Fluid treatment apparatus and process

ABSTRACT

Liquid treatment apparatus comprises at least two chambers being first and second chambers through which a fluid can flow. The two chambers are separated by at least one choke nozzle which has an entrance in the first chamber and an exit in the second chamber. The choke nozzle ( 1 ) comprises a converging section at its entrance ( 3 ), a throat section ( 4 ), a backward-facing step ( 5 ) immediately after the throat section ( 4 ), and an exit section at its exit ( 6 ) wherein the exit section ( 6 ) diverges from the step ( 5 ). Similarly constructed mixing nozzles ( 1 ) may be included in the apparatus. The apparatus is especially useful in processes requiring a gas to be entrained in a fluid so that the gas is in the form of very small bubbles that do not tend to coalesce and flash off such as in the dissolution of gold and other precious metals from ore in the removal of arsenic from an ore.

FIELD OF THE INVENTION

The invention relates to fluid treatment apparatus for treating a fluidmedium including a slurry or pulp and, more particularly, to anapparatus and process for enhancing chemical or physical reactionsoccurring in processes by utilizing choked flow and may provideassociated hydraulic cavitation.

Application of the invention includes not only reactions and processesthat take place in a single or multiple fluid phase but also those inwhich one or more fluids and a gas are contacted such as in a process inwhich a liquid and at least a component of the gas partake in a physicalor chemical reaction. Application of the invention thus extends tochemical extraction processes such as the extraction of precious metalsincluding gold in which a slurry or pulp and an oxidizing gas are mixedand subjected to choked flow; the destruction of cyanide to lower levelsof residual cyanide in waste process solutions pulps and slurries; andthe removal of arsenic from process solutions, pulps and slurries.

Numerous other chemical and physical reactions that take place inprocesses which can benefit from utilizing choked flow as provided bythis invention will doubtless fall within the scope hereof.

BACKGROUND OF THE INVENTION

Cyanide is commonly used as a lixiviant to extract gold and otherprecious metals from ore. Milled ore is mixed with a liquid such aswater to form a slurry or pulp, to which calcium cyanide or sodiumcyanide is then added. An oxidizing agent is required for thedissolution of gold and other metals, and atmospheric air is thecustomary source of oxygen gas for use as the oxidant, although oxygengas is also sometimes used.

A problem occurs, however, in ensuring that the oxygen is sufficientlydiffused within the slurry for oxidation to occur so as to recover themaximum amount of gold from the ore. There is a strong resistance to themixing of air or other oxidizing chemical agents with the slurry, whichmay have a consistency of 50% or more solids, and as a result only aportion of the air in the form of bubbles dissolves in the slurry toprovide for oxidation.

The most commonly used method for injecting oxygen into the slurry is touse a lance/nozzle arrangement to inject air or oxygen into a tank orvessel containing an agitator. The shear from the agitation is then usedto disperse the gas within the tank. A disadvantage of this method,however, is that the shear and Reynolds Numbers generated from themixing action of the agitator are relatively low. Large bubbles are thuscreated, which tend to quickly flash off, resulting in low gas hold-up,low dissolved gas levels and low utilization efficiencies.

Other methods involve pumping the slurry through a pipe withback-pressure and one of a gas injection through a pipe or a lance, gasinjection through slots, and gas injection through porous media, in eachcase relying on the turbulence within the system to break down the gasinto bubbles.

Although these systems generally work better than gas injection into anagitated tank, they have several disadvantages. For example, relativegas hold-up and utilization efficiencies are still relatively low; thereis high wear requiring frequent change outs; and the gas to be injectedhas to be pressurized to above the back-pressure of the system.

Another method involves the use of venturis or eductors, which create ameasure of suction to draw in the gas into the slurry. Disadvantages,however, are that the system is not under pressure; larger bubbles arecreated which can flash off; and the gas hold-up and gas utilizationefficiencies are relatively low.

There is therefore a need for an alternative apparatus and process for,inter alia, promoting the diffusion of a gas into slurry or forgenerating hydraulic cavitation in a process fluid.

In this specification, a fluid is to be regarded as including a liquidsubstance which may also comprise solid material, such as a pulp orslurry as well as entrained gas bubbles or even air. The liquid may bewater or any other liquid, and the solid material may include milled orcrushed ore, heavy metals, water contaminants, effluent, sewage,cellulose and so forth.

SUMMARY OF THE INVENTION

According to the invention, there is provided fluid treatment apparatuscomprising at least two chambers being first and second chambers throughwhich a fluid can flow, the two chambers being separated by at least onechoke nozzle which has an entrance in the first chamber and an exit inthe second chamber, wherein the choke nozzle comprises a convergingsection at its entrance, a throat section, a backward-facing stepimmediately after the throat section, and an exit section at its exitwhich opens into the second chamber.

The choke nozzle is of the general nature of a venturi with theentrance, throat section and exit section each being of circular shapein cross-section and wherein the exit section preferably diverges.

The fluid treatment apparatus is especially configured to toleratechoked flow conditions through the choke nozzle. The diameter of thethroat section of the choke nozzle is selected so as to choke the flowof fluid flowing through the choke nozzle under normal operatingconditions. The design of the choke nozzle therefore varies according tothe required volumetric flow rate and the properties of the liquid to betreated. Typically, more volatile liquids choke at a lower linear speedof as low as about 5 m/s whilst water and slurries in water choke at amuch higher linear velocity in the region of about 25 m/s.

The chambers may be arranged with the first chamber vertically above thesecond chamber and wherein some additional choke nozzles or mixingnozzles can optionally be arranged in inlet or transfer passages withtheir axes generally horizontal as opposed to vertical.

The fluid treatment apparatus may include mixing nozzles for mixingfluids and especially gasses into a process fluid either before itenters a choke nozzle or after it leaves a choke nozzle, or both, andwherein the mixing nozzle has a converging section at its entrance, athroat section, a backward-facing step immediately after the throatsection, and an exit section at its exit which opens into a downstreamchamber. In the instance of water as the base fluid, a typical linearvelocity through the mixing nozzle would typically be between 3 and 12m/s, most preferably between 8 and 10 m/s

The converging section of each nozzle may have a cone angle of from 1 to35 degrees, more preferably of from 15 to 30 degrees; and mostpreferably about 30 degrees.

The backward-facing step may extend radially outwards beyond the throatsection by a distance that generally depends, at least to some extent,on the throat diameter and, especially in the case of a choke nozzle, isabout 3 to about 10% of the diameter of the throat, preferably 4 to 8%,and most preferably about 4.5-5%. In respect of smaller diameter throatsthis represents a step of about 1 to 4 mm, for example about 2 to 3 mm.

The exit section preferably diverges to serve as a diffuser section andmay have a cone angle of from 1 to 8 degrees, more preferably of from 2to 8 degrees, and most preferably of from 4 to 8 degrees. Even morepreferably, the cone angle is about 4 degrees.

The apparatus may be arranged for diffusing a gas in a liquid (includinga slurry or pulp) in which instance the second chamber may include oneor more gas inlets, the inlet(s) being positioned with their axesextending transversely relative to the axis of the nozzle so that gasmay be urged towards a flow that is generally tangential relative to thenozzle with a consequent swirling action.

An exit section to a nozzle may have additional successivebackward-facing steps along the exit section.

The apparatus may be configured to be connected in fluid communicationwith a reactor for separating, purifying, leaching or oxidizing one ormore constituents of the fluid.

The invention also provides apparatus comprising at least first, secondand third chambers in which a first arrangement of a single choke nozzleor a row of more than one choke nozzle as described above extend fromthe floor of a first chamber into a second chamber; a fluid inletleading into the first chamber, the fluid inlet being positionedtransversely with respect to a direction of fluid flow through the chokenozzles; and a second arrangement of a single choke nozzle or a row ofmore than one choke nozzle as described above extends from a floor ofthe second chamber into the third chamber, wherein the entrances to thenozzles of the second and downstream arrangements are directly in linewith the exits of the nozzles from an immediately upstream arrangement;a gas or fluid inlet leading into each of the second and third chambers,each inlet being positioned transversely with respect to the chokenozzles and in line with or slightly downstream of the nozzle exits; anoptional fourth and additional chambers interconnected by arrangementsof a single choke or mixing nozzle or a row of more than one choke ormixing nozzle as described above.

The apparatus may further include an additional chamber above or betweensuccessive chambers described above, the additional chamber having agenerally tangential outlet through which the fluid can flow, the outletbeing generally U-shaped so that fluid is returned tangentially into achamber below that has a single choke nozzle or a row of more than onechoke nozzle in its floor. One or more arrangements of choke nozzles asdescribed above may be positioned in the inlet, preferably near to wherethe inlet enters the chamber below the additional chamber. Gas or fluidinlets may lead into the inlet immediately downstream of the exits ofthe nozzles in the inlet.

The invention also provides a process for enhancing chemical or physicalreactions occurring in processes by utilizing choked flow, the methodcomprising passing a fluid through a choke nozzle comprising aconverging section, a throat section, a backward-facing step immediatelyafter the throat section and an exit section wherein the directionalflow, angular velocity, centrifugal acceleration and straight lineacceleration of the fluid create conditions providing choked flowthrough the choke nozzle.

The invention further provides a process for diffusing a gas in a fluid,the process comprising generating bubbles in the fluid that acceleratesthrough the throat section and then causing the bubbles to implode andform multiple smaller bubbles. Implosion of the bubbles may occur in theexit section which is preferably a diverging section or in a regiondownstream of the choke nozzle. The process may additionally comprisethe step of injecting a gas transversely and preferably generallytangentially into or onto a fluid jet which exits a nozzle at a point offluid discharge, thereby entraining the injected gas in the fluid andimparting a swirling motion to the fluid.

The bubbles which are formed by the process of implosion are preferablyless than 50 micrometres in size; preferably less than 1 micrometre; andeven more preferably less than 1 nanometre, and are sufficiently smallto be retained in the fluid.

A fluid jet exiting one nozzle may be directed into a nozzle immediatelyadjacent, and so on, thereby increasing opportunities within the processfor implosion of bubbles and further cavitation.

The process may be part of a process for separating gold and othermetals from ore, and more particularly, the process may be to ensurethat oxygen or air is sufficiently diffused within a slurry of milledore, water and calcium cyanide or sodium cyanide so as to sufficientlyoxidize the ore for reduced cyanide consumption and/or improved metalleaching and/or to facilitate flotation of the gold particles from theore.

The invention further provides a process for reducing the amount ofcyanide in a cyanide-containing fluid, the process comprising the stepsof adjusting the pH and Eh (oxygen reduction potential measured in mV)of the fluid in an apparatus as described above; and oxidizing cyanidein the fluid by carbon catalysis.

Adjusting the pH and Eh may be performed with an Eh modifier such as acombination of SO₂ and air or oxygen and a catalyst such as coppersulphate. Other Eh modifiers, such as peroxide, manganese dioxide,sodium hypochlorite, potassium permanganate, potassium dichromate orozone may also be used. Oxidizing cyanide may be performed usingactivated carbon or activated charcoal. The process may be performed ina single vessel or in two or more vessels.

The fluid may contain arsenic or a derivative thereof and the process ofthe invention may be aimed at causing the arsenic to dissolve into thefluid; and thereafter precipitating the dissolved arsenic out of thefluid in a stable form. The fluid from which the arsenic has beenprecipitated may be subjected to a further treatment to remove a metalof value from the fluid.

In order that the above and other features of the invention may be morefully understood various embodiments of the invention will now bedescribed with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional elevation of a choke nozzle which can be used inmany different embodiments of the invention;

FIG. 2 is a similar sectional elevation of a mixing nozzle havingmultiple axially spaced backward-facing steps downstream of the throatsection which can be used in many different embodiments of theinvention;

FIG. 3 is an elevation of one embodiment of an apparatus according tothe invention that is particularly aimed at enhancing gas diffusionwithin a fluid and showing two choke nozzles and two coaxial mixingnozzles in dotted lines;

FIG. 4 is a cross-sectional view of the apparatus shown in FIG. 1 takenalong line II to II;

FIG. 5 is a schematic sectional elevation of an alternative embodimentof an apparatus according to the invention using additional arrangementsof nozzles;

FIG. 6 is a schematic sectional plan view taken along line VI to VI ofthe embodiment of apparatus according to the invention shown in FIG. 5;

FIG. 7 is a schematic sectional elevation of an alternative embodimentof an apparatus according to the invention using a single axial mixingnozzle as its inlet;

FIG. 8 is a schematic sectional elevation of an alternative andsimplified embodiment of apparatus according to the invention;

FIG. 9 is a schematic diagram of the progression of a cavitation bubbleimploding close to a fixed surface generating a jet of the surroundingliquid;

FIG. 10 is a schematic diagram showing how sonoluminescence develops inthe direction from left to right;

FIG. 11 shows a low velocity jet that exhibits intermittent gasentrainment;

FIG. 12 shows a high velocity jet that exhibits how gas entrapment canoccur through turbulence within a jet of fluid exiting a choke nozzleand through the shear layer surrounding the jet exiting the chokenozzle;

FIG. 13 illustrates how gas entrapment can occur through splashing offluid out of a receiving cup of a nozzle;

FIG. 14 shows a diagram of flow through a de Laval nozzle, showingapproximate flow velocity (v), together with the effect on temperature(T) and pressure (P);

FIG. 15 illustrates gas entrapment by a droplet entering a stationarypool of liquid;

FIG. 16 is a block diagram of a possible two-stage process for usingapparatus of the present invention in association with a single tank;

FIG. 17 is a graph showing results of a practical test for demonstratingthe efficacy of the invention as applied to the leaching of gold;

FIG. 18 is a graph showing the reduction in cyanide consumption in thetest on which FIG. 17 is based; and,

FIG. 19 is a graph showing results of a practical test for demonstratingthe efficacy of the invention as applied to the dissolution of arsenicfrom a process solution.

DETAILED DESCRIPTION OF THE INVENTION

In the process of the invention in which a gas is to be dispersed in afluid, the gas is injected into the fluid such that ultra-fine bubblesare formed, preferably below 1 micrometre in diameter and even morepreferably in the picometre diameter range, such that the tiny bubblesbehave like solid spheres in the liquid and do not coalesce or flashoff. The creation of ultra-fine bubbles increases the gas hold-up in thefluid; increases the mass transfer of gas into fluid; accelerateschemical reactions; and facilitates the flotation of ultra-fineparticles.

Although the invention is described herein in detail for the recovery ofgold from ore and for the dissolution of oxygen into a slurry or pulp ofmilled ore, water and cyanide, it will be apparent to a person skilledin the art that the invention could have many other applications. Theseinclude the pre-oxidation of mineral pulps; accelerated leaching ofvarious metal values in the minerals industry, e.g. gold, platinum groupmetals and base metals such as copper, cobalt, nickel, zinc, manganeseand lead, as well as uranium; for partial or total sulphide oxidation ofvarious minerals, e.g. in the treatment of refractory gold ore; for thedestruction of cyanide and for arsenic remediation in the gold industry;for the treatment of acid mine drainage; for water treatmentapplications; for applications in the paper and pulp industry; forapplications in the biodiesel industry; for conditioning and ultra-finebubble generation in the flotation industry; and for gas scrubbing.

As mentioned above, one of the problems with existing methods forinjecting gas into a liquid or suspension is the low linear velocities(under 10 m/s) inherent to these systems, which limits the shear andmixing and hence also the bubble size.

A predicted bubble size (diameter) for a fluid velocity of 10 m/s hasbeen calculated to be between 80 and 100 micrometres (microns). Even ifit were possible to increase fluid velocities to 25 m/s, bubble sizewould still only be around 50 micrometres.

The process of the present invention, on the other hand, results in theformation of bubbles which are smaller than 50 micrometres, preferablyin the nanometre or even picometre range. This can be achieved byinitially generating bubbles in the 50 micrometre size range throughshear, and subsequently imploding the bubbles down to nanometre orpicometre range by harnessing the energy of cavitation.

Inertial cavitation is a process where a void or bubble in a liquidrapidly collapses, producing a shock wave (FIG. 9). Since the shockwaves formed by cavitation are strong enough to significantly damagemoving parts, cavitation is usually an undesirable phenomenon. However,in the present invention, conditions favorable for cavitation aredeliberately created and the energy released during cavitation isharnessed and utilized to create nanometre or picometre size bubbles(“nano-bubbles” or “pico-bubbles”), to dissolve gases and to promotechemical reactions that would otherwise not occur or would occur veryslowly (as free radicals are generated in the process due todisassociation of vapours trapped in the cavitating bubbles).

Hydrodynamic cavitation describes the process of vaporization, bubblegeneration and bubble implosion which occurs in a flowing liquid as aresult of a decrease and subsequent increase in pressure. Cavitationwill only occur if the pressure declines to a point below the saturatedvapour pressure of the liquid. In pipe systems, cavitation typicallyoccurs either as the result of an increase in the kinetic energy(through an area constriction) or an increase in the pipe elevation.

Hydrodynamic cavitation can be produced by passing a liquid through aconstricted channel at a specific velocity or by mechanical rotationthrough a liquid. In the present invention, a constricted channel andthe specific geometry of the system create a combination of pressure andkinetic energy enabling a hydrodynamic cavitation cavern downstream ofthe local constriction to generate high energy cavitation bubbles.

The process of bubble generation, subsequent growth and collapse of thecavitation bubbles results in very high energy densities, resulting invery high temperatures and pressures at the surface of the bubbles for avery short time. The overall liquid medium environment, therefore,remains at ambient conditions.

The invention may be implemented using a wide variety of different chokenozzles and mixing nozzles in which gas bubbles are formed in a fluid byaccelerating the fluid through the choke nozzle one variety of which isshown in FIG. 1. The choke nozzle (1) has a fluid inlet (2), aconvergent entry cone (3), a throat section (4) where thecross-sectional area of the choke nozzle is a minimum at the narrow endof the convergent entry cone, a backward facing step (5) immediatelydownstream of the throat section, and a somewhat divergent exit cone ordiffuser section (6) with a fluid outlet (7). The entry cone is angledat from about 10 to about 40 degrees, more particularly from about 15 toabout 35 degrees, and even more particularly from about 25 to about 35degrees, and most particularly at about 30 degrees.

The diameter of the throat section may be selected so as to choke theflow of fluid so that the velocity of any bubbles in the fluid becomessonic in the throat section. The backward facing step (5) can have astep height of about 1 to about 4 mm, and more particularly of about 2to about 4 mm in the instance of smaller diameter throats which h fallswithin the range of about 4.5-5% of the throat diameter. The diffusersection (6) has an angled wall with an included angle of from about 1 toabout 9 degrees, more particularly of from about 2 to 8 degrees, andeven more particularly of from about 4 to 8 degrees with a particularpreference being about 4 degrees. The choke nozzle surface can be roughor pitted. The choke nozzle can be lined with a wear resistant material,such as fused or reaction bonded SiSiC, alumina, HDPE, polyurethane orrubber, a liner being indicated by numeral (8).

In use entrained gas is accelerated past the backward facing step (5),which creates high speed eddies and turbulence within the jet of fluid,resulting in ventilated void bubble formation with subsequent implosion.Bubble implosion is further assisted by the diverging angle of thediffuser section (6) which increases the local (static) pressure in thechoke nozzle as the nozzle diameter increases. Depending on the gas, itmay change phase and liquefy at the point of highest compression.

FIG. 2 of the drawings of the other hand illustrates a mixing nozzlethat is very much elongated and the entry cone (11) of each mixingnozzle is as described above. The entry cone connects to a very muchlonger throat section (12), the length of which is equivalent to fromabout 3 to about 15, and more particularly from about 7 to about 15,diameters of the throat section. Immediately downstream of the throatsection is a first backward facing step (13) with a step height in therange of about 2 to about 25 mm, and more particularly about 4 to about25 mm. There can be a number of subsequent backward facing steps, inthis instance two, additional backward facing steps (14, 15), axiallyspaced at distances equivalent to about 1 to about 10, and moreparticularly about 3 to about 10, diameters of the preceding backwardfacing step. The backward facing steps create a diffuser section with anincluded angle typically of from about 2 to about 30 degrees, and moreparticularly of from about 4 to about 30 degrees. The fluid velocity inthe throat of the mixing nozzle may be between 3 and 12 m/s, mostpreferably between 8 and 10 m/s. The mixing nozzle may have a lining andbe encased as described above.

In each instance, air or other gases, or even liquids, can be injectedinto the fluid at various points such as at the point of fluid dischargefrom the nozzle (assisted by the slight vacuum created by the fluidflow), where it further ventilates the voids and is broken down intosmall bubbles by the implosion of the voids in the highly turbulentregion downstream of the nozzle. The break-up of the fluid greatlyincreases the contact area between the fluid and the gas to furtherenhance dissolution of oxygen in the fluid. The gas injection may betangential and will then result in a swirling action of the fluid, soaiding mixing and generating centrifugal acceleration. Reagents may alsobe injected into the fluid at this point to ensure maximum mixing andreaction.

By accelerating the fluid through a choke nozzle described above, theangular velocity of the fluid can be around 240 000 rpm and thecentrifugal acceleration can be around 60 000 g (g being theacceleration due gravity) at a point close to the centre of the exit ofthe choke nozzle (around 1 mm from the centre). This, coupled with thestraight line acceleration (10 000 g) through the choke nozzle, createsextreme cavitation conditions within the choke nozzle with ventilatedvoid bubbles spreading from the outer circumference (owing to thestraight line acceleration) to the inner core (owing to the centrifugalacceleration).

Thus, nanometre and even picometre size bubbles can be generated bycreating a vacuum bubble by accelerating the fluid to drop theinstantaneous pressure to below the vapour pressure of the fluid, socreating a void bubble; ventilating the void bubble with a gas; andimploding the void bubble by increasing the instantaneous pressure toabove the vapour pressure of the fluid to form a multitude of tinybubbles of an enhanced size.

This acceleration is achieved by one or more of straight lineacceleration through the choke nozzle from about 0.4 m/s to about 25 m/sto generate about 10 000 g (g being the acceleration due to gravity);centrifugal acceleration with angular velocities around 240 000 rpm togenerate around 60 000 g at a point near the centre of the choke nozzle(around 1 mm from the centre); centrifugal acceleration of around 60 000g as a result of eddy formation created by the backward facing steps inthe choke nozzle; and, acceleration due to gravity by height difference(geodetic height).

The acceleration has the effect of “tearing” holes in the liquid to formvoids which are ventilated and imploded. The void can seed onhydrophobic particles in the fluid, on existing microscopic voidsalready in the fluid, or on surface irregularities of the solid surfacesthat provide “leading edges” for cavitation.

The overall effect of the fluid moving through the choke nozzle is thatof an ultrahigh speed swirling jet with ultrahigh speed eddies thatcavitate from both its straight line as well as its centrifugalacceleration.

The turbulence within the fluid jet is also an important factor forfacilitating gas entrainment as the free jet from one nozzle plungesinto the receiving cup or entry cone section of a nozzle below.

Referring to FIG. 12, the speed of the gas is subsonic as it is drawninto the choke nozzle, but it becomes sonic as it is compressed andpasses through the point of narrowest diameter. As it passes into theregion of the backward facing step where the diameter abruptlyincreases, the gas will expand and accelerate to supersonic speeds,generating a shock wave (sound wave) within the jet of fluid. This soundwave would have the effect of causing further cavitation in the jet and,in an extreme case, even breaking up the fluid into a coarse spray togreatly increase surface area for maximum contact with the surroundinggas. As gas is entrained and carried away by the fluid flow, more gas isdrawn into the fluid creating a suction effect.

Although a pressurized gas would not necessarily be required for gasentrainment to take place, it would be preferred owing to the higherresultant gas velocities in the fluid and the possibility of generatingsupersonic gas flows through the nozzles.

Sonoluminescence may occur in the process of the present invention,owing to the shockwaves generated by the gas reaching supersonic speedsand the inertial cavitation in the diffuser sections of the nozzles.FIG. 10 shows the progression from left to right about the upper echelonof a bubble followed by slow expansion and thereafter quick and suddencontraction followed by the emission of light.

Reverting now to the practical implementation of the invention, FIGS. 3and 4 show one arrangement in which a series of axially spaced nozzles(21, 22, 23, 24) are mounted coaxially in tubular apparatus (25). Thefirst nozzle is a mixing nozzle (21) followed by two successive axiallyspaced choke nozzles (22, 23) with a final mixing nozzle (24). In thisinstance there are four tangential gas inlets (26) in the throat of thefirst mixing nozzle (21) and additional gas inlets (27) that are alsotangentially arranged at the outlet (28) from the mixing nozzle (21).

The two choke nozzles (22, 23) each have four tangentially arrangedinlets (29) to feed air or other fluid into of the throat (30) of eachchoke nozzle. FIG. 4 shows clearly the tangential nature of the gasinlets.

FIGS. 5 and 6 illustrate another arrangement of nozzles according to theinvention in a more complex apparatus. In this arrangement the apparatushas a Tee type of inlet pipe (31) leading into a first chamber (32). Theinlet pipe may have one or more points for pressure measurement and gasand/or liquid injection (not shown). The first chamber (32) is typicallya vertical cylindrical pipe with a length of from about 0.3 m to about 1m, more particularly from about 0.4 m to about 1 m, and even moreparticularly from about 0.6 m to about 1 m. The first chamber (32) andinlet pipe (31) can be manufactured from HDPE, steel lined with rubber,polyurethane or any other suitable material.

A roof section (33) of the first chamber (32) can be flanged to allowfor removal for maintenance purposes. At least one choke nozzle, and inthis instance two choke nozzles (34) of the type shown in FIG. 1 arelocated in a floor (35) of the chamber, leading to a second chamber (36)which is similar to the first chamber. A similar arrangement of chokenozzles (37) is positioned in the floor of the second chamber with itsaxis on the centre line of the upstream choke nozzles (34) and arespaced apart so that the distance between the exit of an upstream nozzleand the upper portion of the downstream nozzle is equivalent to fromabout 1 to 3, and more particularly about 2 to 3 diameters of theupstream nozzle exit.

Additional chambers with choke nozzles or mixing nozzles may besimilarly arranged in a succession below those described above with thenozzles being positioned one below the other. In the wall of eachchamber, in line with or slightly below the exit point of each nozzle,there is typically at least one inlet for the addition of one or moregases or liquids to the chamber preferably in a direction that resultsin swirling.

A further chamber (41) with a height of from about 0.4 m to about 1 m,and more particularly from about 0.6 m to about 1 m receives the fluidfrom the last of a succession of nozzles. This further chamber (41) isclosed at its base but has a pair of opposite tangential outlets (42)located in the side wall. Those tangential outlets (42) lead to yet afurther in line chamber (44), via a conduit (45) laterally offset fromthe first and second chambers, and a return tangential inlet (46) thatmay have a choke nozzle (47), which is of the same type described above.The choke nozzle (47) is typically positioned within the return inlet(46) as close as possible to the in line chamber (44). There may be aplurality of choke nozzles arranged in parallel depending on the flowrate to be accommodated.

In the wall of inlet (46), at or about a point where the exit of thenozzle (47) is positioned, there is typically at least one inlet (48)for the addition of one or more gases or liquids. The height of in linechamber (44) can be from about 0.4 m to about 1 m, and more particularlyfrom about 0.8 m to about 1 m. The in line chamber (44) has a closedroof and has choke nozzles (51) of the type described above in itsfloor, leading to yet a further chamber. A succession of chambers (52)may follow as described above with gas inlets (53) provided, as may berequired, at the outlets of the choke nozzles. The final set of nozzlesmay be mixing nozzles of the elongated variety described above withreference to FIG. 2. They may be positioned from about 2 to about 10,and more particularly from about 3 to about 10, nozzle exit diametersaway from the exits from the upstream choke nozzles.

The mixing nozzles discharge into a relatively large chamber (54)compared to the previous chambers from which an exit conduit (55)extends. The length of the exit conduit is typically from about 0.4 m toabout 1 m, and more particularly from about 0.5 m to about 1 m. The exitconduit may feed tangentially or in Tee fashion into an outlet chamber(56) that has a bottom discharge outlet (57).

Rubber bellows or elephant hose (not shown) may be installed at anyinterface between pipework feeding fluid to the apparatus of theinvention and discharging the fluid away from the apparatus. The rubberbellows would absorb unwanted vibration and so assist with protectingthe integrity of welds or joins and the sturdiness of the apparatus.

In use, a fluid of milled ore, water and calcium cyanide or sodiumcyanide may be fed into the first chamber (32) through the inlet pipe(31). The velocity of the fluid at its entry into the chamber should bein the range of from about 1.5 m/s to about 25 m/s, and moreparticularly in the range of about 2.5 m/s to about 25 m/s. At a pointjust before the entry point the back-pressure of the fluid should beabout 3 to about 10 bar, and more particularly about 5 to about 10 bar.Gases or other liquids can be injected into the fluid at or near thispoint through entry points described above. The gases or liquids shouldbe pressurized to pressures of from about 5 to about 20 bar, and moreparticularly from about 10 to about 20 bar, and can be injected eitherdirectly into the fluid or via a nozzle arrangement.

The gases or liquids introduced downstream should also be pressurized topressures of from about 5 to about 20 bar, and more particularly fromabout 10 to about 20 bar, and can be injected either directly into thefluid or via a nozzle arrangement, or could alternatively beself-aspirated owing to the vacuum created by the fluid flowing throughthe nozzle.

Gas entrapment can occur within or between the nozzles via one or moreof the following mechanisms through turbulence within the jet exiting anozzle (FIG. 12); through the shear layer surrounding the jet exitingthe nozzle (FIG. 12); through the recirculating eddies between the jetexiting the nozzle and the liquid/suspension in the receiving pool ofthe nozzle located below it; between the wall of the receiving cup ofthe nozzle located below the jet and the liquid/suspension in the cup ofthe receiving nozzle; by splashing of liquid/suspension out of thereceiving pool (FIG. 13).

Other embodiments of the invention are depicted in FIGS. 7 and 8. FIG. 7shows a simplified embodiment of the invention, incorporating only aplurality of choke nozzles as illustrated in FIG. 5 for a more compactdesign. FIG. 7 also shows a coaxial inlet (61) in which there isinstalled a mixing nozzle (62). An inlet chamber (63) communicatesdirectly with an arrangement of tangential outlets (64) and tangentialinlet (65) with reference to FIG. 5. The choke nozzles are indicated bynumeral (66).

FIG. 8 shows a more simplified arrangement in which there are simplythree layers of choke nozzles (67) between a Tee inlet (68) and a Teeoutlet (69).

The process and apparatus of the present invention can be arranged toresult in an increased rate of cyanide destruction compared to knownprocesses.

The commercially accepted process for cyanide destruction utilises acombination of SO₂ and air with a CuSO₄ catalyst in a well-agitated tankto oxidise cyanide to cyanate and so “destroy” the cyanide. One of theshortcomings of this process is the high reagent consumption. Someminerals also compete for the SO₂, resulting in unsuccessful destructionof the cyanide to the accepted industry standard of 50 ppm.

The reactor of the present invention can be used in the followingtwo-stage process in the first of which the pH and Eh adjustment iscarried out utilizing the reactor (with air or oxygen injection into thereactor) in addition to an Eh modifier such as SO₂/air and a catalystsuch as copper sulphate. Other Eh modifiers such as peroxide, manganesedioxide, sodium hypochlorite, potassium permanganate, potassiumdichromate or ozone may also be utilized. In a second stage oxidation ofcyanide by carbon catalysis utilizing activated carbon such as that usedin a carbon in leach plant is carried out.

In its simplest form, the two stage mechanism described above can beperformed simultaneously in a single tank, with appropriate screeningtechnology utilized to prevent the carbon from entering the reactor.Pumping of carbon through the reactor would result in undesirableincreased carbon abrasion and breakage, with loss of potentially goldbearing carbon to tailings.

When the SO₂/air with copper sulphate catalyst, together with carboncatalyzed cyanide destruction, are used in the above process, itrepresents a hybrid between the known INCO process (as described in U.S.Pat. No. 4,537,686) and the Maelgwyn process (US Publication Number2010/0307977). This hybrid process uses significantly less reagents thanrequired for the INCO process (as little as one tenth of the INCOreagents).

The hybrid process also employs the catalytic effect of activated carbonto ensure the destruction of cyanide via two different mechanisms(SO₂/air and activated carbon catalysis). The process is able to reducethe residence time required in the Maelgwyn process, with a positive Ehvalue for successful destruction, and also results in the simultaneousleaching and recovery of precious metals such as gold, by adsorptiononto the carbon.

More importantly, the hybrid process as described above can be performedin a single stage, as opposed to the multiple stages required for theMaelgwyn Process.

FIG. 16 shows a flow diagram of how the reactor of the present inventioncan be integrated into a carbon in leach plant. A reactor (71) accordingto the invention can be installed in the first two tanks (72) to reducereagent consumption and accelerate leach kinetics. This could free upthe last two tanks to be utilized for cyanide destruction and arsenicand heavy metal removal. In addition to catalyzing the leach reaction,the carbon in the last tank would also ensure that soluble gold lossesare kept to a minimum.

A test was conducted using a standard SO₂/air cyanide destructionprocess as described in U.S. Pat. No. 4,537,686 in a single 60 minutestage with standard reagent addition (2:1 stoichiometric ratio of SO₂ tocyanide) (Table 1), and this was compared to the hybrid process of thepresent invention as described above for the same time period (Table 2).From similar weak acid dissociable cyanide starting values, the processof the present invention resulted in lower final cyanide values than theSO₂/air process and utilized only one tenth of the reagents used in theSO₂/air process.

TABLE 1 Cyanide destruction using a commercial SO₂/Air process SO2/AirReagent/CN Stage 1 Free CN ppm 94 WAD CN ppm 96 Thiocyanate ppm 5.8Total CN ppm 97 Test Slurry Flow Rate mL/min 16 Conditions Solid FlowRate g/min 10.1 Stage Residence Time min 60 pH 9.25 Eh mV 136 4-5 HoursSMBS g/t 992 CuSO4•5H2O g/t 258 Lime g/t 488 Free CN ppm 0.03 WAD CN ppm0.07 Thiocyanate ppm 4.7 Total CN ppm 0.64

TABLE 2 Cyanide destruction using the process of the present invention.Gold Ore Hybrid Detox Data Stage 2 Free CN ppm 26 WAD CN ppm 98Thiocyanate ppm 4.2 Total CN ppm 41 Test Slurry Flow Rate mL/min 9.1Conditions Solid Flow Rate g/min 5.49 Stage Residence Time min 30 pH8.68 Eh mV 101 6-7.5 Hours SMBS g/t 118 CuSO4•5H2O g/t 226.8 H₂SO₄ g/t44 Free CN ppm 0.04 WAD CN ppm 0.03 Thiocyanate ppm 1.2 Total CN ppm0.08

These tests were repeated under the same conditions but using a feedmaterial of different mineralogy (Tables 3 and 4). The commercialSO₂/air process was unable to render a final cyanide value of below 50ppm, which is an industry regulated standard for effluent discharge.

TABLE 3 Cyanide destruction using a commercial SO₂/air process SO2/AirReagent/CN Stage 1 WAD CN ppm 163 Total CN ppm 173 Test Slurry Flow RatemL/min 16 Conditions Solid Flow Rate g/min 10.1 Stage Residence Time min60 pH 9.25 Eh mV 136 4-5 Hours SMBS g/t 1580 CuSO4•5H2O g/t 60 Lime g/t530 WAD CN ppm 77 Total CN ppm 87

TABLE 4 Cyanide destruction using the process of the present inventionGold Ore Hybrid Detox Reagent/CN Stage 2 WAD CN ppm 164 Total CN ppm 173Test Slurry Flow Rate mL/min 16 Conditions Solid Flow Rate g/min 10.1Stage Residence Time min 30 pH 8.52 Eh mV 88 4-5 Hours SMBS g/t 250CuSO4•5H2O g/t 60 H₂SO₄ g/t 51 WAD CN ppm 12 Total CN ppm 17

The process of the present invention thus not only has the potential tobe significantly more cost effective and environmently friendly thanexisting technologies, but is also potentially technically superior.

Subsequently an industrial scale plant trial was carried out over aperiod of 20 days of the first 10 days being run with the reactor ofthis invention is switched off and the second 10 days being run with thereactor switched on. The gold residue results are shown in FIG. 17 andthe cyanide consumption results are shown in FIG. 18. There is adistinct improvement of a reduction in the residual gold in the residueof naught 0.32 g/t and an improvement in the cyanide consumption of 84g/t which was a reduction of 36%.

The foregoing relates mostly to the introduction of gas such as anoxidizing gas into a fluid. However, there are other applications of theinvention in which the introduction of gas is not necessary, and one ofthese is in the destruction of arsenic removal.

Arsenic occurs naturally in underground rock in a stable form whichdoesn't dissolve in water. However, when the rock is mined and the oreis brought to the surface and into contact with air, the arsenic isconverted to an unstable form which readily dissolves in water. Thus,wastewater from mining operations frequently contains highconcentrations of arsenic. As arsenic is toxic to both humans andanimals, steps need to be taken to reduce the risks of contamination ofgroundwater from the wastewater, and the internationally acceptableupper limit for arsenic in wastewater from mining operations ispresently set at 0.1 ppm. Mining operations which produce wastewaterwith higher levels of arsenic generally need to line their tailings damswith a layer of plastic to prevent any possible contaimination of theenvironment. This is not only very expensive, but also does not preventor reduce the creation of toxic waste.

A reactor of the present invention can be used to leach the arsenic outof the native mined mineral ore and into solution within a relativelyshort period of a few hours. The dissolved arsenic can then bepreciptated out of the solution as scorodite, a stable form of arsenicwhich does not dissolve in water and is therefore not toxic, or ascorodite-like mineral.

The arsenic remediation can be carried out as an initial step prior tometal extraction utilising a mechanically agitated tank with a reactoron recirculation with air or oxygen addition into the reactor (ozone mayalso be used). In order to effect the leaching of the arsenic, thefollowing reagents may be employed sodium metabisulphite (SMBS) orcaustic soda (NaOH); and hydrochloric acid (HCl) or sulphuric acid(H₂SO_(4);)

Ferric chloride can be used to effect the precipitation of arsenic asstable scorodite or scorodite-like minerals.

Two tests were conducted on gold ore containing reactive Gersdorffiteand nickeline, both of which are unstable forms of arsenic.

The first test was conducted under the following standard gold leachingconditions to serve as a control or base case: 24 hour leach time; 5kg/t NaCN addition; 10 g/l carbon addition; 40% solids; test conductedin agitated vat.

The second test utilized the same leach conditions as the first, butwith a prior arsenic leaching and precipitation stage conducted underthe following conditions: 800 g/t SMBS; 300 g/t copper sulphate; 2 kg/tHCl; 50 g/t phosphoric acid; 50 g/t alum; 300 g/t ferric chloride; 4hour residence time;

40% solids; 10 reactor passes with oxygen addition (one pass equals onevessel volume turnover); test conducted in agitated vat.

The results of these tests are shown in Table 5. JR691 is thecontrol/base test and JR689 is the test incorporating the arsenicleaching and precipitation according to the invention.

TABLE 5 Arsenic and residual gold values after gold leaching Au Au Au AuAu Au As Au ppm ppm ppm ppm ppm ppm mg/l ppm rpt1 rpt2 rpt3 rpt4 rpt5Average soln JR689 0.65 0.59 0.58 0.58 0.60 0.62 0.60 <0.10 24 hrs RESIJR691 0.95 0.98 0.97 1.00 0.94 1.03 0.98 1.30 24 hrs RESI

The process incorporating the arsenic leaching and precipitation stepresulted in arsenic in solution values below detection at less than 0.1ppm arsenic at the end of the leach. The control/base test, however,showed 1.30 ppm arsenic at the end of the leach. This is significant, asthe arsenic values of the control/base test do not comply withenvironmental regulations whereas the process incorporating the arsenicleaching and precipitation step of this invention is environmentallycompliant.

In addition, the process incorporating the arsenic leaching andprecipitation step had a gold residue 0.38 g/t lower than thebase/control test, which provides a significant economic benefit and avaluable boost to gold production levels.

Thus, the reactor of the present invention can be used to leach out andprecipitate arsenic from minerals, so renderring the arsenic more stableand resulting in little or no further leaching of arsenic when depositedon tailings storage facilities. This results in compliance withregulations relating to arsenic levels in groundwater and discharge intonatural waterways. The process can also provide higher levels of goldrecovery.

The results of an industrial scale test are shown in FIG. 19 for fourdifferent conditions namely an untreated condition and four differentconditions involving a process according to the invention equating to 3reactor passes for a duration of 4 hours with different additions offerric chloride and sodium metabisulphite (SMBS), as reflected in FIG.19. The additions were 2.5 kg/t ferric chloride and 240 g/t SMBS; 1.75kg/t ferric chloride and 2.23 kg/t SMBS; 1.00 kg/t ferric chloride and3.5 kg/t SMBS; and 0.00 kg/t ferric chloride and 5.5 kg/t SMBS.

Numerous other processes can doiubtless be carried out using theapparatus and process of this invention

1. Fluid treatment apparatus comprising at least two chambers beingfirst and second chambers through which a fluid can flow, the twochambers being separated by at least one choke nozzle which has anentrance in the first chamber and an exit in the second chamber, whereinthe choke nozzle comprises a converging section at its entrance, athroat section, a backward-facing step immediately after the throatsection, and an exit section at its exit which opens into the secondchamber wherein the choke nozzle is configured to promote cavitationresulting from choked flow under normal operating conditions.
 2. Fluidtreatment apparatus as claimed in claim 1 in which the convergingsection, the throat and the exit section are each of circular shape incross-section and the exit section diverges.
 3. Fluid treatmentapparatus as claimed in claim 1 in which mixing nozzles are included inthe apparatus for mixing fluids either before they enter a choke nozzleor after they leave a choke nozzle, or both and wherein a mixing nozzlehas a converging section at its entrance, a throat section, abackward-facing step immediately after the throat section, and an exitsection at its exit which opens into a downstream chamber.
 4. Fluidtreatment apparatus as claimed in claim 1 in which the convergingsection of a nozzle has a cone angle of from 1 to 35 degrees.
 5. Fluidtreatment apparatus as claimed in claim 4 in which the cone angle isfrom 15 to 30 degrees.
 6. Fluid treatment apparatus as claimed in claim1 in which the backward-facing step of a choke nozzle extends radiallyoutwards beyond the throat section by 3 to 10% of the diameter of thethroat.
 7. Fluid treatment apparatus as claimed in claim 6 in which thebackward-facing step extends radially outwards beyond the throat sectionby from 4 to 8%.
 8. Fluid treatment apparatus as claimed in claim 1 inwhich the exit section diverges with a cone angle of from 1 to 8degrees.
 9. Fluid treatment apparatus as claimed in claim 8 in which theexit section diverges with a cone angle of from 2 to 8 degrees. 10.Fluid treatment apparatus as claimed in any one of the claim 1 in whichthe apparatus is arranged for diffusing a gas in a fluid in whichinstance the second chamber includes one or more gas inlets having theiraxes extending transversely relative to the axis of a nozzle so that gasmay be urged towards a flow that is generally tangential relative to thenozzle or has a consequent swirling action.
 11. Fluid treatmentapparatus as claimed in claim 1 in which a diverging exit section to anozzle has additional successive backward-facing steps along the exitsection.
 12. A process for enhancing chemical or physical reactionsoccurring in processes by utilizing choked flow, the process comprisingpassing a fluid through a choke nozzle comprising a converging section,a throat section, a backward-facing step immediately after the throatsection and an exit section wherein the directional flow, angularvelocity, centrifugal acceleration and straight line acceleration of thefluid create conditions providing cavitation consequent on choked flowthrough the choke nozzle.
 13. A process as claimed in claim 12 in whichthe process includes diffusing a gas in a fluid and the processcomprises generating bubbles in fluid that accelerates through thethroat section and causing the bubbles to implode and form multiplesmaller bubbles.
 14. A process as claimed in claim 12 in which theprocess forms part of a process for separating gold and other metalsfrom ore in which oxygen or air is diffused within a slurry of milledore, water and calcium cyanide or sodium cyanide so as to sufficientlyoxidize the ore for reduced cyanide consumption and/or improved metalleaching and/or to facilitate flotation of the gold particles from theore.